Commercial indium tin oxide (ITO) films are used in a variety of optoelectronics, digital displays, touch panels, organic light-emitting diodes, optical coatings, gas sensors, electrochemical cells, and solar cells also termed photovoltaic (PV) devices. ITO is known for its high transmittance of visible light (80-90%) while maintaining high sheet conductivity (8-12 Ω/sq) and thermal stability (T<400° C.) (see ref. 2). Drawbacks to ITO include the limited freedom and high cost of deposition. ITO films are commonly deposited under specific atmospheric conditions by atomic sputter deposition onto glass or quartz at temperatures above 450° C., or the conventional solution process shown in FIG. 1. Recently, a report on solution based ITO demonstrated spin-cast films of indium nitrate and tin chloride dissolved in a solvent with a fuel and oxidizer (see ref. 1). Upon heating to temperatures ranging from 250-450° C., an exothermic reaction is initiated that produces extreme local temperatures hot enough to produce crystalline ITO (the combustion process shown in FIG. 1). In an iterative layer-by-layer process alternating between heating and spin coating, the ITO film can be deposited to the desired thickness. These films suffer from low transmittance, however, and contain a yellow colored solid material. In experiments with solar cells, they were also found to have lower conductivity than reported in the literature. In order to produce high quality ITO films that are both transparent to visible light (>80%) and conductive (<400 Ω/sq), a new formula for solution deposition of ITO is desired.
An answer to this problem would allow a solution ITO procedure to be applied to solar cells to produce the first all-solution inorganic solar cell. Solution processing of inorganic solar cells has been investigated by several groups (see refs. 3-9). Notable is the ITO/CdTe/CdSe/Al structure first demonstrated by Alivisatos et al. (2005) (ref. 7) where both CdTe and CdSe were spin coated onto ITO from solution of nanocrystals. Since then, other groups have included solution processed PbS, PbSe, and CuInS2 among others. These inorganic nanocrystal cores were synthesized with an organic capping ligand to facilitate the material solubility. The inorganic core contains the cadmium chalcogenide semiconductor with a narrow band gap which can absorb visible light and produce an electron/hole pair as a result of the absorbed photon. These charges can be collected by the ITO or Al contact to produce light driven power. While some of these devices employ a solution processed contact (i.e. AgNWs (ref. 10), Ag paint (ref. 11), graphite (ref. 12), each of these examples; however, still use vacuum sputtered transparent conductive oxides (TCOs), (refs. 4, 6, and 10-18), e-beam evaporated metal contacts (ref. 4, 6, and 13-18) or metal contact substrates where the device was built on patterned copper (ref. 19). It is not believed that a complete PV device has been deposited via solution onto non-conductive substrates.